BGE: How to change camera's focal length?
These two lines are all you need to change the active camera's focal length. 

cam = bge.logic.getCurrentScene().active_camera
cam.lens = 90.0 #or whatever value you want


bge.logic.getCurrentScene().active_camera gets the active camera in the current scene.
Then cam.lens is accessing the lens attribute of the camera defined in line one.

Read through the docs about the game camera, and you will see other properties you can get or set.

Do note that because field of view and focal length are related you can not set both lens and fov at once. Which ever value you set last will be the value that is used, and the other will be calculated for it.